ELSD diffuser

ABSTRACT

An evaporative light scattering detector (1) has a solvent nebulizer (4) through which an atomised spray of solute solvent solution passes, a heated evaporation chamber (5) and a detector system (6). A diffuser trapping device (7), supported by fine wires, is positioned in the evaporation chamber(5) at about two fifths of the height of the chamber from the top. The diffuser trapping device (7) is made from randomly coiled stainless steel ribbon which gives it a large surface area. The detection system (6) is positioned at the bottom of the evaporation chamber through which a beam of collimated light passes and is scattered by the solvent and detected by a light sensitive device (14). The diffuser trapping device (7) aids evaporation of the solvent and prevents large particles from travelling further down the evaporation chamber (5).

The instant application is a continuation-in-part of Ser. No.09/051,825, pending, filed Oct. 5, 1998, which is a 371 ofPCT/GB97/02360, filed Sep. 3, 1997.

This invention relates to a diffuser trapping device for use with anebulisation system to produce a uniform atomised spray for useparticularly, though not exclusively, in evaporative light scatteringdetectors (ELSD) for liquid chromatography.

Evaporative light scattering detectors are highly sensitive liquidchromatography detectors for non-volatile solutes dissolved in avolatile liquid stream (solvent). An ELSD operates in threestages--firstly nebulisation of the solvent occurs wherein the solventor solvent/solute solution is atomised into a dispersion of droplets bya venturi jet operated by a jet of compressed air or an inert gas, suchas nitrogen. Secondly the atomised spray passes into an evaporationchamber under the influence of the nebuliser gas flow, which may be fanassisted, that directs the atomised spray down the evaporation chamberand vents the exhaust at the rear of the instrument. The third stage isthat of detection. Collimated light is passed through the instrumentperpendicularly to the direction of gas flow at the base of theevaporation chamber. A light trap is positioned opposite the lightsource to eliminate internal reflections inside the body of theinstrument. When a pure solvent is evaporated only its vapour passesthrough the light path and the amount of light scattered is small andconstant. The presence of a non-volatile solute causes a particle cloudto pass through the light path resulting in light scattering. Thescattered light generates a signal response from a photomultiplier orother light sensitive device which is provided in the detection system.The quantity of light detected is dependent on both concentration andparticle size distribution of the solute.

Evaporative light scattering detectors are used as concentrationdetectors in many liquid chromatography techniques such as gelpermeation chromatography (GPC), high performance liquid chromatography(HPLC) and supercritical fluid chromatography (SFC).

Large droplets or large particles of solute that have not been fullynebulised and which may be present in the atomised spray can giveexcessive scattering to create a dramatic increase in background noiseor noisy signal responses respectively and a decrease in the overallsensitivity of the detector. These large droplets or particles can arisefrom inadequate nebulisation and inefficient drying of the droplets.

By large droplets or particles it is meant those having a diameter equalto or greater than the wavelength of incident light.

With the above problems in mind we have now developed adiffuser-trapping device for use with a nebuliser system which willreduce the presence of undesirable large droplets or particles therebyreducing background noise and improving the sensitivity of the detector.

Accordingly from a first aspect the present invention provides anevaporative light scattering detector comprising a solvent nebuliser, aheated evaporation chamber and a detection chamber into which isdirected a collimated light beam normal to the flow of nebulised solventand a light sensitive device for detecting scattered light wherein adiffuser-trapping device is positioned within the evaporating chamber ata depth of between one-third and two-thirds of the height of theevaporating chamber and extends substantially across the full diameterof the chamber.

Accordingly from a second aspect the present invention comprises adiffuser-trapping device suitable for positioning within the heated areaof the evaporation chamber of the ELSD of the first aspect of thepresent invention comprising a three dimensional highly porous networkof inert material preferably formed from a thermally conductive materialwhich may have a fibrous construction.

The expression "highly porous" refers to a structure having a porosityof between 50 and 99% calculated from the void volume or free spacerelative to the total volume occupied by the diffuser trapping device.

The position of the diffuser trapping device depends upon the diameterof the spray exiting the nebuliser and is positioned at such a levelwithin the evaporation chamber that the atomised particles are partiallyevaporated before they reach the diffuser trapping device. The optimumposition being such that the spray hits the diffuser trapping devicebefore it hits the heated walls of the evaporation chamber and is burntout.

If the diffuser trapping device is positioned too close to the nebuliserthe spray soaks into the diffuser which affects the temperature controland therefore the operation of the apparatus.

In a preferred embodiment the diffuser-trapping device is positioned ata depth of two fifths of the height of the evaporation chamber measuredfrom the top of the chamber.

The diffuser-trapping device may comprise coiled ribbon or filaments ofthermally conductive corrosion resistant metal most preferably the metalis thin stainless steel.

The metal ribbon is preferably coiled randomly, for example to form ahighly porous tortuous network with a very high void content but withouta direct line of sight pathway through the network. Otherdiffuser-trapping systems can be made from a random bundle of fibres orsintered metal or a series of overlapping porous meshes arranged so asto provide no line of sight passage through the network.

It is believed that the diffuser-trapping device functions in severalways:

Firstly the diffuser-trapping device prevents the larger and heavierparticles or droplets from reaching the light scattering area bytrapping them in the network whereas the smaller lighter particles,which are attached to the air flow, can travel through the tortuousnetwork to the detector.

Secondly the diffuser provides a large surface area which contacts theheated walls of the evaporation chamber and gets hot giving a heatedsurface area which results in more efficient evaporation of the solventin the droplets and particles containing solvent. It also aids thevaporisation of the large droplets and dries the particles trapped onthe diffuser-trapping device.

Thirdly the diffuser directs the flow of atomised solute/solventsolution closer to the heated walls of the evaporation chamber whichagain aids evaporation of the solvent.

Each of the above functions of the diffuser-trapping device help toreduce the number of large droplets or particles which reach the lightscattering chamber which, as has already been mentioned, reducesbackground noise and improves the sensitivity of the evaporative lightscattering detector.

The performance of an ELSD instrument incorporating a diffuser of thepresent invention can be further improved by the incorporation of meanspositioned below the diffuser to cause the particle plume exiting thediffuser to be brought into closer contact with the inner hot surface ofthe evaporation tube. Such a means may be in the form of a rod having asliding fit into the evaporation tube and containing at least onehelical groove, such a rod hereinafter referred to as a helix. A rodcontaining four parallel helixes, a four start helix, is preferred. Thehelix may be formed of any suitable material such as glass, plasticsmaterial, ceramic or metal, stainless steel being preferred.

It is preferred that the helix includes an integral spring machined intoone end to hold the helix in position in the evaporation tube.

Further enhancement of the instrument is achieved by positioning afurther (post) diffuser immediately following the helix which acts torecombine the streams exiting the helix. The randomisation of thestreams provided by the post diffuser prevents pulsation in the opticalchamber thereby avoiding a cyclic pattern on the baseline. The postdiffuser may be formed of the same material as the first, main diffuser.

The diffuser-trapping device of the present invention has manyapplications and can be used in conjunction with a variety of systems toreduce the number of undesirable large solvent or solvent/soluteparticles in a nebulised or atomised spray including, but not limitedto, evaporative light scattering detection.

The following applications are examples of use and improvements providedby the diffuser-trapping device as applied to an ELSD and in no waylimit the applications to other systems, other types of ELSD or limitthe applications of the ELSD.

1. High Performance Liquid Chromatography Applications (HPLC)

In carbohydrate chemistry, refractive index (RI) detectors are commonlyused, however they are very unstable and temperature sensitive. The ELSDof the present invention overcomes these problems by providing anenhanced sensitivity and being stable to changes in externaltemperature.

In Peptide chemistry Ultraviolet absorption (UV) detectors are used,however the use of acetonitrile and trifluoroacetic acid as solvents cancause baseline deflections or drift and make integration difficult. TheELSD of the present invention gives level and non-drifting baselinethroughout the chromatogram.

Many polar water soluble polymers are undetectable by UV detectors andshow poor detection with an RI detector, however the ELSD of the presentinvention can detect these polymers to high sensitivity levels andmaintains a level baseline.

The ELSD of the present invention can also be used in the detection ofsuch substances as pharmaceuticals, surfactants, triglyceridespreservatives and fat soluble vitamins.

2. Gel Permeation Chromatography Applications (GPC)

The ELSD of the present invention can operate at temperatures of up to200° C. making it ideal for substances such as polyolefins. Enhancedsensitivity and maintaining a stable baseline are also beneficial in theanalysis of polyolefins where other detection is difficult.

Embodiments of the present invention will now be described by way ofexample only with reference to the figures of which:

FIG. 1 is a cross section of an evaporative light scattering detectorincorporating the present invention;

FIG. 1a is a view of the diffuser-trapping device of the presentinvention;

FIG. 2 is a chromatogram for glucose obtained from an ELSD with andwithout a diffuser-trapping device;

FIG. 3 is a chromatogram for polystyrene Mw 260000 g/mol obtained froman ELSD with and without a diffuser-trapping device;

FIG. 4 is a chromatogram for glucose showing an overlay of the resultsobtained for glucose from an ELSD having the diffuser-trapping devicepositioned at three different heights;

FIG. 5 is a helix for use with the diffuser-trapping device of FIG. 1a;

FIG. 6 is an illustration of an evaporation tube used in the ELSDinstrument of FIG. 1 incorporating a helix and post diffuser; and

FIG. 7 chromatograms for glucose with and without the use of a helix.

FIG. 1 shows one example of an evaporative light scattering detector 1having a solvent inlet 2, consisting of a small bore stainless steelcapillary tube which can be attached to a column outlet. An inlet 3 forcompressed air or inert gas is provided between the solvent inlet 2 anda solvent nebuliser 4 which is of a known type. A narrow cylindricalevaporation chamber 5 of length 330 mm and diameter 19 mm extendsvertically downwards from the nebuliser 4 to the detection system 6. Adiffuser-trapping device 7 is positioned inside the evaporation chamber5 at a depth of h (100 mm), and is supported by fine stainless steelwires (not shown). A cylindrical casing 8 surrounds the nebuliser 4 andevaporation chamber 5. An exhaust chamber 9 is located in the base ofthe instrument, below the detection system 6, having a small fan (notshown) and an exhaust vent 9a.

The detector system 6 and exhaust chamber 9 are housed within acylindrical casing 15. Two tubes 11 and 13 enter the casing 15 at 120°to each other and perpendicular to the evaporation chamber 5. Each tube11, 13 has an aperture 10, 16 in its end remote from the casing 15. Alight source 17 is provided outside tube 11 in front of aperture 10 andinside tube 11 a collimater 19 is positioned such that the light fromsource 17 passes through it. A photomultiplier detector 14 is providedat the end of tube 13. A light trap 12 is positioned opposite the lightsource.

FIG. 1a shows the diffuser-trapping device 7 which is constructed from aribbon of stainless steel 18 which is randomly coiled to give anamorphous ball. The ribbon is 0.04 mm thick and 0.59 mm wide and thediffuser trapping device has a weight of 0.9 gms.

In use the solvent is fed directly from the outlet of a chromatographycolumn or other suitable means into the ELSD through a solvent inlet 2.The solvent flows into the nebuliser 4 where a venturi jet operated bycompressed air, or an inert gas, entering the ELSD through thecompressed gas inlet 3 atomises the solvent into a dispersion ofdroplets which then flow down into the evaporation chamber 5. Theevaporation chamber 5 is heated by three band heaters situated aroundthe exterior of the chamber (not shown) so that the droplets of atomisedsolvent evaporate during their passage down the chamber 5. The dropletshave to pass through the diffuser 7 which is supported at a depth h(being 100 mm) from the top of the evaporation chamber. Thediffuser-trapping device 7 has a large surface area and is heated bycontact with the heated evaporation chamber in the vicinity of a bandheater with the result being that it makes evaporation more efficientand acts as a trap to block large particles of solvent or solvent andsolute from travelling further down the evaporation tube.

The small particles or droplets which are attached to the air passthrough the diffuser-trapping device and flow down the remaining lengthof the evaporation chamber 5 and into the detector system 6. Light fromsource 17 passes through aperture 10 and through the collimater 19 intube 11 the resultant collimated light then passes through the particlecloud flowing out of the evaporation chamber 5. The particle cloudscatters the light beam and scattered light travels along tube 13, outof aperture 16 into a photomultiplier detector 14 which generates asignal response. A light trap 12 eliminates internal reflections fromthe direct light beam within the instrument body. Exhaust venting occursthrough exhaust chamber 9 and outlet 9a.

The effectiveness of the diffuser-trapping device 7 can be investigatedby monitoring the signal to noise ratio of a solute response from thedetector. A higher signal to noise ratio indicates a more effectivediffuser-trapping device. Evaporative light scattering detectionexperiments using standard detectors not having a diffuser-trappingdevice as well as those incorporating the present invention were carriedout on the following substances using the conditions specified.

    ______________________________________                                        Test probe 2 Glucose Mw 180 g/mol analysed in water                           ______________________________________                                        Chromatography Conditions:                                                    Column:          PL-GFC 8 μm 300Å 300 × 7.5 mm                   Concentration:   1 mg/ml                                                      Injection Volume:                                                                              50 μl                                                     Solvent Flow:    1.0 ml/min                                                   Detector Conditions:                                                          Evaporation Temperature:                                                                       80° C.                                                Gas Flow:        7 L/min                                                      ______________________________________                                    

    ______________________________________                                        Test probe 1 Polystyrene; MW 260000 g/mol, Mn 100000 g/mol                    analysed in tetrahydrofuran.                                                  ______________________________________                                        Chromatography Conditions:                                                    Column:         Plgel 5 μm MIXED-C 300 × 7.5 mm                      Concentration:  1 mg/ml                                                       Injection Volume:                                                                             50 μl                                                      Eluent Flow:    1.0 ml/min                                                    Detector Conditions                                                           Evaporation Temperature:                                                                      40° C.                                                 Gas Flow:       7 L/min                                                       ______________________________________                                    

FIGS. 2 and 3 show the traces obtained when the above experiments werecarried out, a significant reduction in background noise can be seenwhen the diffuser-trapping device of the present invention is used inconjunction with an ELSD. Areas of the baseline in both figures havebeen magnified to show the effect of the presence of thediffuser-trapping device on the background noise (areas A and B in FIG.2 and area C in FIG. 3).

Experiments were conducted to determine the optimum weight of thediffuser-trapping device by monitoring the signal to noise ratio (S/N)of the peak obtained for test probe 2 (Glucose) for diffuser-trappingdevices with a weight ranging from 0.7 to 0.9 g. The results of thisexperiment are shown in table 1 below.

    ______________________________________                                        Weight of Diffuser                                                                         Signal (S)   Noise (N)                                           (g)          (mV)         (mV)     S/N                                        ______________________________________                                        0            448.3        2.14     209                                        0.701        734          0.69     1063                                       0.775        819          0.66     1240                                       0.800        819          0.60     1365                                       0.850        723          0.29     2424                                       ______________________________________                                    

The results show that a diffuser-trapping device reduces the noisesignificantly and the signal to noise ratio increases with increasingweight of the device up to approximately 1 gm.

The effect of the depth of the diffuser trapping device in theevaporation chamber can also be investigated by monitoring the signal tonoise ratio at different diffuser depths.

Evaporative light scattering detection experiments using standarddetectors incorporating the diffuser trapping device of the presentinvention at various positions were carried out under the followingconditions. The performance was monitored from the signal (peak height)of a glucose and the short term noise measured on the base line.

    ______________________________________                                        Chromatography Conditions:                                                    Column:          PL-GFC 8 μm 300Å 300 × 7.5 mm                   Eluent           Water (HPLC grade)                                           Test Probe       Glucose (0.1% w/v)                                           Injection Volume:                                                                              50 μl                                                     Eluent flow rate 1 ml/min                                                     Detector Conditions:                                                          Temperature:     80° C.                                                Gas              Air (dried and filtered through                                               0.2 μm membrane)                                          Gas Flow:        7.5 L/min                                                    ______________________________________                                    

Gas Air (dried and filtered through 0.2 μm membrane)

Gas flow 7.5 L/min

The signal to noise ratio (S/N) of the test probe peak (glucose) wasdetermined with the diffuser trapping device positioned at a depth of 40mm, 90 mm, 120 mm, 155 mm and 260 mm from the top of the evaporationchamber. The results are shown in Table 2 below.

    ______________________________________                                                  Signal (S)    Noise (N)                                             Depth (mm)                                                                              (mV)          (mV)     S/N                                          ______________________________________                                        40        No result                                                           90        447           ˜0.5                                                                             894                                          120       656           0.097    6763                                         155       591           0.118    5008                                         260       517           0.126    4103                                         ______________________________________                                    

FIG. 4 shows an overlay of the traces obtained when the above experimentwas carried out.

The results show that the diffuser trapping device reduces the noisesignificantly and increases the signal to noise ratio with increasingdepth of the position of the diffuser-trapping device from the top ofthe evaporation chamber up to approximately 120 mm.

Positioning the diffuser trapping device at a depth of 40 mm from thetop of the evaporation chamber caused a heater error on the detector dueto an excessive build up of water.

As noted above the performance of the ELSD instrument can be furtherimproved by the incorporation of a helix in the evaporation chamber 5.

FIG. 5 provides an isometric view of a suitable four-start helixcomprising a body portion 30 formed of stainless steel containing fourhelix grooves 31 1 mm in depth in the outer surface thereof andincluding an integral spring clip 32 at one end thereof.

A form of evaporation chamber incorporating the helix is shown in FIG.6. An evaporation chamber 33 having a length of 162 mm and an internaldiameter of 8 mm is provided with a main diffuser trapping device 34positioned 81 mm from the top of the chamber of comprising 0.45 gm ofsteel wool in the form of a cylinder 20 mm in length and 7.7 mm incross-section. A four-start helix 35, 56.5 mm long and 7.7 mm indiameter and having 1 mm deep helical grooves is positioned immediatelybelow and in contact with diffuser 34. The helix is maintained inposition by the integral spring clip 36. A second; post, diffuser 37consisting of 0.25 gm steel wool formed into a 10 mm plug is looselypushed after the helix.

The effectiveness of the presence of the helix in the evaporationchamber is illustrated by comparing the baseline noise with and withoutthe presence of the diffuser. Three instruments of the same batch weretested with water using the conditions given below with only thetrapper-diffuser 34 installed. The helix 35 and post diffuser 37 werethen fitted and the instrument re-tested. The results are shown in Table1 below.

    ______________________________________                                        Chromatography Conditions                                                                      Detector Conditions                                          ______________________________________                                        Column: PLaquagel OH 30 5 μm                                                                Evaporation temperature: 120° C.                      7.5 × 300 mm                                                                             Nebuliser temperature: 90° C.                         Eluent: Water (18 Mohm,                                                                        Gas flow rate: 1.5 SLM                                       filtered to 0.2 μm)                                                                         (Nitrogen)                                                   Flow rate: 1 ml/min                                                           Test probe: Glucose                                                           Injection mass: 12.5 μg                                                    (0.25 mg/ml, 50 μl)                                                        ______________________________________                                    

                  TABLE 1                                                         ______________________________________                                        Signal to Noise improvements when the helix is added to the                   evaporation chamber                                                           Before              With Helix                                                A           B       C       A     B     C                                     ______________________________________                                        Noise (mV)                                                                            0.136   0.137   0.223 0.063 0.070 0.0825                              Signal (mV)                                                                            642     792     596   606   694   538                                SNR     4720    5781    2672  9617  9914  6520                                ______________________________________                                    

The results clearly show the improvement in the measured baseline noiseand thus the overall increase in-signal-to noise ratio. This effect isfurther illustrated in FIG. 7 taken from the figures of instrument A.The reduction in the number of spiking on the baseline indicates thatthe solute particles were more effectively dried when the helix wasinstalled. The small reduction in peak height confirms this effect sincethe droplets are reduced in size.

What is claimed is:
 1. An evaporative light scattering detector (ELSD)comprising:a solvent nebuliser, a heated evaporation chamber, adetection chamber into which is directed a collimated light beam normalto the flow of nebulised solvent, a light sensitive device for detectingscattered light, a porous, heat-conductive, diffuser-trapping devicepositioned within said evaporation chamber at a depth of between onethird and two thirds of the height of said evaporation chamber andextending substantially across the full diameter of said chamber, and ahelix formed of an inert material positioned below and adjacent thediffuser-trapping device and extending substantially across the fulldiameter of the evaporation chamber.
 2. The ELSD according to claim 1wherein the helix is a four-start helix.
 3. The ELSD according to claim1 wherein the helix includes an integral spring clip at one end thereofto retain the helix in the evaporation chamber.
 4. The ELSD according toclaim 1 wherein the helix is formed of a material selected from, inertplastic, glass ceramic or metal.
 5. The ELSD according to claim 4wherein the helix is formed from stainless steel.
 6. The ELSD accordingto claim 1 further including a post-diffuser adjacent and below thehelix.
 7. The ELSD according to claim 1 and claim 6 wherein the diffuserand post diffuser both comprise a three dimensional highly porousnetwork of inert material.
 8. The ELSD according to claim 7 wherein theinert material is coiled ribbon or filaments.
 9. The ELSD according toclaim 8 wherein the inert material is a corrosion resistant metal. 10.The ELSD according to claim 9 wherein the inert material is stainlesssteel.
 11. An evaporative light scattering detector (ELSD) comprising asolvent nebuliser, a heated evaporation chamber and a detection chamberinto which is directed a collimated light beam normal to the flow ofnebulised solvent and a light sensitive device for detecting scatteredlight including a diffuser trapping device including a pre-diffuserpositioned within said evaporation chamber at a depth of between onethird and two thirds of the height of said evaporation chamber andextending substantially across the full diameter of said chamber, afour-start helix formed of inert material positioned adjacent and belowthe pre-diffuser and extending substantially across the full diameter ofthe chamber and a post-diffuser positioned adjacent and below the helixand extending substantially across the full diameter of the evaporationtube, the pre and post diffusers comprising a three dimensional highlyporous network of inert material.
 12. An ELSD according to claim 11wherein the diffusers are formed of ribbons of stainless steel.
 13. AnELSD according to claim 12 wherein the helix is formed from stainlesssteel.